The interaction of colicin K with membranes will be studied from two points of view. In the first, the release of newly synthesized colicin from the colicinogenic cell will be examined. Pulse-labeled cytoplasmic colicin appears in a different electrophoretic form from membrane-bound or extracellular colicin, and the meaning of this will be explored. The second point of view will examine the interaction of colicin, which is normally a water-soluble protein, with liposomes and cell membranes. The conformation of the protein is expected to differ in the two environments. The possibility that the colicin receptor activates the colicin by proteolysis or another mechanism will be tested.